Signal generators are known for generating analog signals representative of various waveforms, which typically, are voltage signals. Such signal generators typically comprise a circuit which outputs a voltage signal representative of the desired waveform. By applying the output voltage signal to a cathode ray tube, or other suitable visual display unit, a waveform representing the output signal is displayed. In their simplest such circuits may include a switch circuit which periodically switches a supply voltage Vdd on and off, and the switched voltage is applied to an output terminal which alternates between the supply voltage and ground, thereby producing an analog output voltage signal representative of a square waveform. The frequency of the output signal is selected by selecting the frequency with which the switch circuit switches on and off the supply voltage to the output terminal. The mark/space ratio of the output signal is selected by selecting the durations of the periods during which the supply voltage is switched to the output terminal, and is isolated therefrom. However, a problem with such signal generators is that in general, they are suitable only for generating an output signal representative of a square waveform which oscillates between a fixed maximum value, which typically, is the value of the supply voltage Vdd, and a fixed minimum value, which typically is ground. Without the addition of relatively complex circuitry, it is not possible to vary the maximum and minimum voltage values between which the output signal swings.
Signal generators which comprise a digital to analog converter (DAC), for example, a voltage DAC, are also known. The DAC outputs an analog voltage signal which is representative of the desired waveform. A plurality of digital words which correspond to voltage values of the waveform are sequentially applied to the DAC from an external circuit in an appropriate sequence, and at appropriate time intervals so that the DAC outputs an analog output signal which is representative of the desired waveform.
While such signal generators comprising a DAC are suitable for generating an analog output signal representative of a desired waveform, they suffer from a number of disadvantages. Firstly, they tend to be inefficient, in that they do not utilise the DAC to its maximum efficiency, and secondly, they require external circuitry for storing digital words which correspond to desired voltage values of the output signal. The external circuitry requires programming in order that the digital words are applied to the DAC in the appropriate sequence and at the appropriate time intervals so that the analog signal outputted by the DAC is representative of the desired waveform. A further disadvantage, and a particularly serious disadvantage is that the digital words must be transferred from the external circuitry to the DAC, for in turn loading into the DAC, and this can significantly slow down the operation of the DAC, particularly where the digital link between the external circuitry and the DAC is through a serial interface. This is undesirable.
There is therefore a need for a signal generator for generating an analog output signal representative of a waveform, and in particular, a square or a stepped waveform which overcomes at least some of the problems of known signal generators.
The present invention is directed towards such a signal generator, and the invention is also directed towards a method for generating an analog signal representative of such a waveform.